1. Field of the Invention
The present invention relates to apparatus for a Coriolis mass flow rate meter which is substantially immune to noise, and more particularly, to such a meter that is substantially unaffected by noise that occurs at any frequency different from a fundamental frequency at which the flow tube(s) in the meter vibrates and methods for use in such a meter.
2. Description of the Prior Art
Whenever a fluid flows through a rotating or oscillating conduit, Coriolis forces are produced which are perpendicular to both the velocity of the fluid moving through the conduit and the angular velocity of the rotating or oscillating conduit. The magnitude of these Coriolis forces is proportional to the product of the mass flow rate of the fluid and the angular velocity of the conduit. In general, so-called Coriolis mass flow rate meters measure the mass flow rate of the fluid by sensing oscillatory motion of the conduit that results from the Coriolis forces generated by the moving fluid.
In general, Coriolis forces that appear in these mass flow rate meters are rather small in comparison to other forces that are normally present in the meter, such as momentum forces, inertial forces, pressure forces, and others. Consequently, sensitive and precise instrumentation was often employed in early Coriolis mass flow meters known in the art in order to accurately measure the small Coriolis force effects, such as conduit deflection, which resulted from moderate mass flow rates and reasonable angular velocities. Such instrumentation was usually quite expensive. In addition, the angular velocity of the conduit also had to be accurately measured and controlled in order to determine the mass flow rate of the fluid passing through the conduit as a function of the magnitude of the generated Coriolis forces.
A mechanical configuration and measurement technique which, among other things, avoids the need to measure and control the magnitude of the angular velocity of the conduit and also its sensitivity and, to a reasonable degree, accurately measures the Coriolis force is taught in U.S. Reissue Pat. No. 31,450 (issued to Smith on Nov. 29, 1983 and hereinafter referred to as the '450 reissue patent). This patent discloses a mechanical configuration which incorporates a U-shaped flow tube, devoid of pressure sensitive joints, which has its open ends attached to opposite sides of a manifold. When so counted, this flow tube is capable of being oscillated about an axis oriented perpendicular to the side legs of the U-shaped tube. This axis is located near the tube-manifold interface and is situated in a plane in which the U-shaped tube lies at rest. This plane is hereinafter referred to as the midplane of oscillation. When fluid flows through the mounted U-shaped flow tube, the filled flow tube is forced to oscillate. These oscillations are sufficient to cause the free end of the flow tube to pass through the mid-plane of oscillation and thereby generate a Coriolis force couple which elastically deflects the free end of the flow tube about an axis. This axis is located in the plane of the flow tube midway between and parallel to its side legs. The flow tube is designed to resonantly oscillate about this axis and another axis orthogonal thereto such that the forces which oppose the generated Coriolis forces are predominantly linear spring forces. Consequently, these spring forces cause one of the two side legs of the flow tube to pass through the midplane of oscillation before the other side leg does so. As such, the mass flow rate of the fluid that flows through the flow tube is proportional to the width of the time interval (time delay) occurring between the passage of the respective side legs of the tube through the mid-plane of oscillation. This time interval and, hence, the mass flow rate of the fluid can be measured, within a reasonable degree of accuracy, using optical sensors as disclosed in the '450 reissue patent, or by using electromagnetic velocity sensors, as disclosed in U.S. Pat. No. 4,422,338 (issued to Smith on Dec. 27, 1983).
The '450 reissue patent also teaches the use of a spring arm which extends from the manifold along with the U-shaped flow tube. When this spring arm is sinusoidally driven in opposition to the U-shaped flow tube, the combination of spring arm and U-shaped flow tube operates as a tuning fork. This operation substantially attenuates undesirable vibrations occurring at the tube-manifold and spring arm-manifold interfaces. This attenuation is extremely advantageous for the following reason. In practice, these undesirable vibrations often occur, particularly at the tube-manifold interfaces, with sufficient intensity to effectively mask tube movement caused by the small Coriolis forces and thereby introduce significant errors into the time interval measurements of the passage of the side legs of the U-shaped tube through the mid-plane of oscillation. Because the mass flow rate is proportional to the time interval measurements, these errors inject significant inaccuracies into the measured mass flow rate. Tuning fork operation substantially cancels these undesirable vibrations and thereby significantly increases measurement accuracy. In addition, reducing vibrations that occur at the manifold also decreases long term fatigue effects induced by vibrations that might otherwise occur on the meter mounting structure. The substitution of a second flow tube, having a similar configuration to the first flow tube, for the spring arm, provides an inherently balanced tuning fork structure. The inherent symmetries in such a structure further reduce undesirable vibrations and thereby further increase measurement accuracy. This teaching has been recognized in the design of densimeters wherein measurements of the resonant frequency of filled flow tubes are used to determine the density of fluids in the tubes. See, for example, U.S. Pat. Nos. 2,635,462 (issued to Poole et al during Apr. 1957) and 3,456,491 (issued to Brockhaus during July 1969).
The art also teaches the use of a serial double flow tube configuration in a Coriolis mass flow rate meter. Such a configuration is described in U.S. Pat. Nos. 4,127,028 (issued to Cox et. al. on Nov. 28, 1978); 4,192,184 (also issued to Cox et al on Mar. 11, 1980) and 4,311,054 (also issued to Cox et al on Jan. 19, 1982). Here, incoming fluid sequentially passes through one flow tube, then through an interconnecting conduit and lastly through another flow tube. Unfortunately, series type double flow tube meters possess an inherent drawback: since all the fluid must pass through two flow tubes instead of one, the fluid pressure drop across the meter is greater than that of a non-serial type flow meter. One way to compensate for this increased pressure drop is to increase the pressure at which the incoming fluid is supplied to the meter. Unfortunately, this generally entails increasing the pumping capacity of the entire fluidic system that supplies fluid to the meter.
An alternate configuration involving parallel flow tubes is disclosed in U.S. Pat. No. 4,491,025 (issued to Smith on Jan. 1, 1985 and hereinafter referred to as the '025 patent). Here, incoming fluid is evenly divided between and flows into parallel, illustratively two U-shaped, flow tubes rather than sequentially passing through two serially connected flow tubes. At the output end of each parallel flow tube, the fluid is combined in a drain manifold and from there exits the meter. The two flow tubes are sinusoidally oscillated. As the fluid moves through both flow tubes, Coriolis forces are produced which alternately deflect adjacent legs of the tubes and, in turn, permit time interval measurements to be made in order to determine the mass flow rate of the fluid.
The parallel flow tube design provides significant advantages over the discussed prior art designs that utilize single or serially connected flow tubes. First, each parallel flow tube may be constructed with relatively thin walls which, in turn, provides increased sensitivity. Specifically, as the wall thickness of a flow tube decreases, the mass and rigidity of the tube also decreases which, in turn, increases tube deflection caused by Coriolis forces. Increasing the deflection for a given mass flow rate advantageously increases the sensitivity of the meter. Second, parallel tube flow meters are, in general, operationally more stable than either single flow tube or serial flow tube meters. This occurs because the fluid flowing through both tubes results in a dynamically balanced pair of tuning fork tines, i.e. as the mass of one tine varies due to increased fluid density so will the mass of the other tine. Third, parallel flow tube meters are less sensitive to error-producing external vibrations and, hence, provide more accurate fluid flow measurements than do single tube or serial tube flow meters. This occurs because the time interval measurement sensors can be mounted on the flow tubes without a physical reference to any structure that is immutably fixed with respect to the mid-planes of oscillation for the tubes. Fourth, parallel flow tube meters exhibit less pressure drop across the entire meter than does a serial flow tube meter.
Other parallel and serial flow tube designs known in the art are typified by those shown in U.S. Pat. No. 4,252,028 (issued to Smith et al on Feb. 24, 1981) and 4,660,421 (issued to Dahlin et al on Apr. 28, 1987), the latter patent being assigned to Exac Corporation of Campbell, Calif.
Now, although parallel and serial flow tube designs known in the art provide reasonable performance, the accuracy of these designs is adversely affected by noise. In Coriolis mass flow rate meters, the mass flow rate of a fluid that moves through the meter is only proportional to the time interval (time delay) that occurs at the driving frequency of the flow tube. Hence, in all Coriolis mass flow rate meter designs known in the art--whether serial or parallel, a critical goal of any of these designs is to measure a time interval for tube movement that occurs at only the fundamental frequency at which the tube is being driven. This interval is the time that elapses between the instant one point situated on a side leg of the flow tube crosses a pre-determined location, e.g. a respective mid-plane of oscillation, until the instant a corresponding point situated on the opposite side leg of the same flow tube, crosses its corresponding location, e.g. its respective mid-plane of oscillation.
Unfortunately, in Coriolis mass flow rate meters known in the art--whether using one or more flow tubes in either a serial or parallel configuration, I have discovered that these time delay measurements contain components that occur at frequencies other than the resonant (fundamental) driving frequency of the flow tube. These components disadvantageously inject error into the time delay measurements which, in turn, adversely affects the overall accuracy of the meter.
Specifically, to greatly simplify both the mathematics involved in designing a Coriolis mass flow rate meter and the electronic circuitry that processes the measured time delays, the art has generally assumed that each flow tube, that is used in such a meter, can be accurately modeled as a lumped spring mass system that possesses a single degree of freedom. As a result, this assumption provides a first order approximation of the actual behavior of each flow tube. While this assumption greatly simplifies the mathematics, the assumption over-simplifies the behavior of each oscillating flow tube. In actuality, each flow tube is a continuous mechanical system having many degrees of freedom in which an infinite number of preferential frequencies (modes) can exist. As such, the frequency response of this system will resemble that of a classical filter having a number of spectral lines (response peaks) at increasing frequencies. Certain of these peaks result from bending modes of the tube, others result from torsional modes of the tube and the like. These modal frequencies are often not harmonically related. Therefore, even if each flow tube is driven at its fundamental frequency of vibration, due to the Coriolis forces, the sinusoidal movement of the tube sides will exhibit response peaks at other frequencies in addition to the fundamental driving frequency. The response peaks occurring at these other frequencies tend to corrupt the time delay measurements.
In addition to modal sources, noise occurs from other sources. For example, the sensors used to measure tube motion are generally assumed to operate as linear devices. These sensors are generally either optical devices or electromagnetic velocity sensors which, in reality, are non-linear devices that produce harmonics when excited. Therefore, the output of these transducers are generally corrupted by harmonic components. Electromagnetic velocity sensors, which are typically used in the art, contain coils and/or magnets which are mounted to the flow tube. Such a velocity sensor has characteristics that markedly change with temperature. As such, the signals produced by these sensors also frequently contain harmonic components.
Furthermore, broadband noise is often produced by virtue of the fluid flowing through a flow tube. Specifically, as the mass flow rate increases, the flow becomes increasingly turbulent which, in turn, increases the modal excitation of the vibrating flow tube. This is particularly evident with gaseous flows which often generate acoustical waves within the flow stream that inject significant amounts of noise into the flow tube sensor signals. The amount of this noise is often so large as to render Coriolis meters unsuitable for measuring gaseous flows.
Now, in the Coriolis mass flow rate measurement art, time intervals, occurring between movement of respective side legs of the flow tube, are typically measured using traditional zero (or level) crossing techniques. Unfortunately, if noise, such as harmonics or broadband noise, contaminates the signals produced by either tube sensor, then the phase shift between the two flow tube sensor signals will disadvantageously change from its true value. Inasmuch as the actual phase shift is often a small value, then any such noise may inject a noticeable error component into measured phase shift and thereby into the measurement of the actual mass flow rate of the fluid as it travels through the meter. Often, in an attempt to remove the noise, the zero (or level) crossing detectors are preceded by a bandpass filter, such as a well known Chebyshev or Butterworth type analog filter. Unfortunately, the output of these filters varies with temperature. Moreover, it is difficult to provide two such analog filters that are exactly matched to each other for temperature variations. Consequently, with analog filters, temperature variations will likely inject error into the phase and hence into the mass flow rate measurements. One way to eliminate this error would be to convert the analog signals produced by the flow tube sensors into a stream of digital values, filter these values digitally and re-convert the results back into the analog domain to measure the phase using conventional zero (or level) crossing detectors. Unfortunately, such an approach is quite complex and unnecessarily expensive.
Due to the apparent difficulty and attendant expense associated with adequately removing noise from flow tube sensor signals, it appears that the art has merely accepted the fact that such signals used in Coriolis mass flow rate meters will contain noise, whether harmonic, broadband or otherwise. As a result, the overall accuracy of presently available Coriolis mass flow rate meters and the use of these meters in certain applications have been limited by the presence of noise.
Therefore, a need exists in the art for a highly accurate Coriolis mass flow rate meter and particularly one that is substantially immune to noise. Specifically, a need exists for such a meter that is inexpensive and substantially insensitive to any frequency other than the fundamental frequency at which a flow tube in the meter is driven.